The European Central Bank started buying covered bonds with negative yields as its asset-purchase program reduces the supply of the highly rated debt, according to two people familiar with the matter.

The central bank bought the debt in the past two weeks, said the people, who asked not to be identified because the information is private. The notes were from Germany, one of the people said.

The ECB has bought 69.7 billion euros ($75.5 billion) of covered bonds since October as part of its latest measures designed to stimulus growth in the euro area. The accumulation of assets is driving down yields and the central bank now holds about 15 percent of the market, according to ABN Amro Bank NV.